Monday, November 25, 2013

QGIS Desktop 2 has a easy to use plug-in - Topology Checker, that can be used to identify dangles in linear features. If it is not installed, then it is a simple matter to use the Plug-in Manager to download and install it, as shown below.

Using the Topology Checker to find dangles is straightforward. Just choose the linear feature layer and define the validation rule(s), and run it. An example usage is illustrated below.

Start QGIS 2. Load and display a linear feature layer e.g. segments.

Select Vector | Topology Checker | Topology Checker.

The Topology Checker pane appears on the right.

In the Topology Checker pane, click Configure.

The Topology Rules Settings appear.

In the Current Rules first drop down combo box, choose a linear feature e.g. segments.

The second and/or third combo boxes are enabled.

In the second combo box, choose a validation rule e.g. must not have dangles. Click Add Rule.

Monday, November 18, 2013

There is a Road Graph plug-in that can be installed in QGIS, and used to perform directional and non-directional shortest path analysis. Using the plug-in is quite straight forward, provided the network data is available and topologically clean. The following is an example showing a non-directional shortest path calculation using a linear metro network layer from a SpatiaLite database but it could from any GIS format.

Monday, November 11, 2013

It is possible to split a polygon feature by interactively drawing a line over it in QGIS 2.0 with the Split Feature command. The command doesn't seem to be accessible from the menus and only available as a tool bar item in the Advanced Digitizing tool bar palette. The following shows how to access it to split a polygon.

Monday, November 4, 2013

Once a while I get an unusual request like burning contour vector lines onto GeoTiff images. It's a good thing the open source software GDAL has a gdal_rasterize utility to do the job. Here is an illustrated example of burning vector lines onto a raster image.

Input vector contour lines

Input raster GeoTiff file

Open up a Windows Command Prompt.

Type in the following command, (assuming the GDAL installation bin folder is in the system path).